This invention relates to methods and apparatus for delivering, deploying and retrieving medical endoprostheses, commonly referred to as xe2x80x9cstentsxe2x80x9d. More specifically, the invention relates to delivering a stent composed of shape-memory material on a balloon catheter without using an outer protective sheath, and deploying the stent by introducing temperature-controlled fluid through the balloon catheter to induce a shape change in the shape-memory material, the fluid causing the balloon to expand along with the stent thereby enhancing control over placement of the stent and enabling retrieval of the stent when necessary.
Stents are known in the prior art for maintaining the patency of a diseased or weakened vessel or other passageway. Stents have been implanted, for example, in passageways in the urinary tract system and in the coronary arteries of the endovascular system. Such mechanical prosthetic devices are typically inserted into the vessel, positioned across an affected area and then expanded, or allowed to self-expand, to keep the vessel or passageway open. Effectively, the stent overcomes the natural tendency of the weakened area to close. Stents used in the vascular system are generally implanted transluminally during or following percutaneous transluminal coronary angioplasty (angioplasty or PCTA).
A number of vascular stents have been proposed, including self-expanding stents and expandable stents. Self-expanding stents may be mechanically compressed springs which expand when released, and/or they may be constructed from shape-memory materials including shape-memory polymers and metals such as nickel-titanium (xe2x80x9cNitinolxe2x80x9d) alloys and the like, which have shape-memory characteristics.
In a manner known in the art, a stent formed of shape-memory alloy such as nickel-titanium is formed into a desired expanded configuration and then heated until the metal crystals assume their high-temperature structure known as the beta phase, parent phase or austenite. The stent is then cooled so that the crystals transition to a martensite crystal structure, generally with no change in shape. The material is then formed into a reduced diameter configuration for implantation.
When the temperature of the stent is later raised so that the crystal structure transitions to the parent phase, the shape of the stent returns to the desired expanded configuration. Typically, a material having a phase-change transition temperature in excess of 125xc2x0 F. is chosen to prevent premature expansion of the stent upon exposure to human body temperature (98.6xc2x0 F.). However, materials with lower phase-change transition temperatures may be used, provided the stent is insulated from the human body temperature prior to reaching the location for deployment.
Dotter U.S. Pat. No. 4,503,569 describes a stent comprising a helically wound coil of shape-memory alloy that is placed over the outer wall of a guide catheter. The stent and guide catheter are carried within an outer sheath. At the delivery site, the outer sheath is withdrawn. A hot saline solution is then injected through the inner guide catheter and flows out of the catheter through apertures, so that the fluid bathes the stent and causes it to expand. The patent describes that a balloon may be positioned on one or both sides of the stent to reduce thermodilution of the saline solution and enhance the phase change.
Balko et al. U.S. Pat. No. 4,512,338 which discloses a Nitinol wire stent in the form of a coil which is delivered to the desired site while housed inside of an insulating sheath. When the insulating sheath is withdrawn, the surrounding body temperature causes the stent to expand.
Froix U.S. Pat. No. 5,163,952 discloses shape-memory-polymer tubular and coil stents having an elastic memory which causes them to expand to a predetermined diameter upon exposure to particular conditions. Shape-memory polymer stents are initially formed having the desired expanded diameter. The stents are then physically stretched at elevated temperature causing them to assume a reduced diameter. While under tension, the stents are cooled to below the glass transition phase of the plastic. The stent then remains in the stretched and reduced-diameter configuration until after the stent is raised to above the glass transition temperature at the site desired to be treated. Depending upon the polymer selected and process of manufacture, stents can be formed to expand by different degrees and upon exposure to various conditions including various temperatures.
Various methods have been used to induce the temperature change required to effect the shape-memory characteristic, including intervascular electrical resistive heating elements, R.F., and temperature-controlled fluid boluses injected through a guide catheter around the delivery catheter.
A drawback of previously known delivery systems for shape-memory material stents is the use of a protective sheath to prevent premature expansion at body temperatures and to enhance delivery through the tortuous vessels of the vascular system. Such sheaths increase the cross-sectional profile of the delivery system, necessitating use of a delivery catheter with a larger diameter. The large diameter of the delivery catheter may in turn increase the risk of complications at the patient access site.
The increased cross-sectional profile of the delivery system also detracts from the ability of the device to navigate through tortuous vessels or passageways. The increased cross-sectional profile of the delivery system may make it impossible to deliver a phase-change material stent to the area desired to be treated and may decrease the ability to deliver sufficient contrast material through the guide catheter for enabling precise positioning.
Another drawback of self-expanding stents, both coiled spring stents and phase-change stents, is the inability to control expansion of the stent once the stent loses contact with the guide catheter. In particular, as the stent expands, or is pushed out of a catheter for self-expansion, it moves radially out of contact with the delivery device. Because there no longer is contact between the stent and the delivery device, there is no mechanism to control positioning of the stent location during this critical phase of deployment. Consequently, the stent may move away from the desired site.
Yet another problem is encountered when elevated (or reduced) temperatures are required to cause the phase change in the stent material. For example, in the system described in the Dotter patent, the fluid flowing through the catheter exchanges heat with the vessel wall, and as the fluid flows out of the catheter it is diluted by the blood or other fluids contained in the vessel. Thus, it may be necessary to flush large volumes of liquid through the guide catheter to maintain the required temperature environment around the stent to effect deployment. The injection of such large volumes of hot or cold fluid into the vessel may be injurious or hazardous to the vessel and the tissues through which it passes.
It would therefore be desirable to provide methods and apparatus for delivering a shape-memory material stent to an affected area of a vessel without using a sheath.
It would further be desirable to provide methods and apparatus for effecting the phase change of the stent without perfusing large quantities of fluid into the vessel to be treated.
It would still further be desirable to provide methods and apparatus for maintaining control over the positioning of a phase-change stent during deployment.
It is an object of this invention to provide methods and apparatus for delivering a phase-change stent to an affected area of a vessel without using a sheath.
It is a further an object of this invention to provide methods and apparatus for effecting the phase change of the stent without perfusing large quantities of fluid into the vessel to be treated.
It is still further an object of this invention to provide methods and apparatus for maintaining control over the positioning of a phase-change stent during deployment.
These and other objects of the invention are accomplished in accordance with the principles of the invention by providing a phase-change stent crimped or encapsulated onto the balloon of a balloon catheter having perfusion apertures communicating from the inflation lumen to the exterior of the catheter to enable a temperature-controlled fluid to be used both to pressurize the balloon and induce expansion of the stent. Some of the intended uses for a delivery system in accordance with the present invention include PTCA type stenting, PTA type stenting, graft support, graft delivery, INR use, GI tract use, drug delivery, and biliary stenting.